Lithographic apparatus and device manufacturing method

ABSTRACT

An immersion lithography apparatus includes a liquid supply system configured to supply a liquid to a space through which a beam of radiation passes, the liquid having an optical property that can be tuned by a tuner. The space may be located between the projection system and the substrate. The tuner is arranged to adjust one or more properties of the liquid such as the shape, composition, refractive index and/or absorptivity as a function of space and/or time in order to change the imaging performance of the lithography apparatus.

This application claims priority from European patent application EP03256499.9, filed Oct. 15, 2003, which is incorporated herein in itsentirety.

FIELD

The present invention relates to a lithographic projection apparatus anda device manufacturing method.

BACKGROUND

The term “patterning device” as here employed should be broadlyinterpreted as referring to any device that can be used to endow anincoming radiation beam with a patterned cross-section, corresponding toa pattern that is to be created in a target portion of the substrate;the term “light valve” can also be used in this context. Generally, thepattern will correspond to a particular functional layer in a devicebeing created in the target portion, such as an integrated circuit orother device (see below). Examples of such a patterning device include:

-   -   A mask. The concept of a mask is well known in lithography, and        it includes mask types such as binary, alternating phase-shift,        and attenuated phase-shift, as well as various hybrid mask        types. Placement of such a mask in the radiation beam causes        selective transmission (in the case of a transmissive mask) or        reflection (in the case of a reflective mask) of the radiation        impinging on the mask, according to the pattern on the mask. In        the case of a mask, the support structure will generally be a        mask table, which ensures that the mask can be held at a desired        position in the incoming radiation beam, and that it can be        moved relative to the beam if so desired.    -   A programmable mirror array. One example of such a device is a        matrix-addressable surface having a viscoelastic control layer        and a reflective surface. The basic principle behind such an        apparatus is that (for example) addressed areas of the        reflective surface reflect incident light as diffracted light,        whereas unaddressed areas reflect incident light as undiffracted        light. Using an appropriate filter, the undiffracted light can        be filtered out of the reflected beam, leaving only the        diffracted light behind; in this manner, the beam becomes        patterned according to the addressing pattern of the        matrix-addressable surface. An alternative embodiment of a        programmable mirror array employs a matrix arrangement of tiny        mirrors, each of which can be individually tilted about an axis        by applying a suitable localized electric field, or by employing        piezoelectric actuation means. Once again, the mirrors are        matrix-addressable, such that addressed mirrors will reflect an        incoming radiation beam in a different direction to unaddressed        mirrors; in this manner, the reflected beam is patterned        according to the addressing pattern of the matrix-addressable        mirrors. The required matrix addressing can be performed using        suitable electronic means. In both of the situations described        hereabove, the patterning device can comprise one or more        programmable mirror arrays. More information on mirror arrays as        here referred to can be gleaned, for example, from U.S. Pat. No.        5,296,891 and U.S. Pat. No. 5,523,193, and PCT patent        applications WO 98/38597 and WO 98/33096, which are incorporated        herein by reference. In the case of a programmable mirror array,        the support structure may be embodied as a frame or table, for        example, which may be fixed or movable as required.    -   A programmable LCD array. An example of such a construction is        given in U.S. Pat. No. 5,229,872, which is incorporated herein        by reference. As above, the support structure in this case may        be embodied as a frame or table, for example, which may be fixed        or movable as required.        For purposes of simplicity, the rest of this text may, at        certain locations, specifically direct itself to examples        involving a mask and mask table; however, the general principles        discussed in such instances should be seen in the broader        context of the patterning device as hereabove set forth.

Lithographic projection apparatus can be used, for example, in themanufacture of integrated circuits (ICs). In such a case, the patterningdevice may generate a circuit pattern corresponding to an individuallayer of the IC, and this pattern can be imaged onto a target portion(e.g. comprising one or more dies) on a substrate (silicon wafer) thathas been coated with a layer of radiation-sensitive material (resist).In general, a single wafer will contain a whole network of adjacenttarget portions that are successively irradiated via the projectionsystem, one at a time. In current apparatus, employing patterning by amask on a mask table, a distinction can be made between two differenttypes of machine. In one type of lithographic projection apparatus, eachtarget portion is irradiated by exposing the entire mask pattern ontothe target portion at one time; such an apparatus is commonly referredto as a stepper. In an alternative apparatus—commonly referred to as astep-and-scan apparatus—each target portion is irradiated byprogressively scanning the mask pattern under the projection beam in agiven reference direction (the “scanning” direction) while synchronouslyscanning the substrate table parallel or anti-parallel to thisdirection; since, in general, the projection system will have amagnification factor M (generally <1), the speed V at which thesubstrate table is scanned will be a factor M times that at which themask table is scanned. More information with regard to lithographicdevices as here described can be gleaned, for example, from U.S. Pat.No. 6,046,792, incorporated herein by reference.

In a manufacturing process using a lithographic projection apparatus, apattern (e.g. in a mask) is imaged onto a substrate that is at leastpartially covered by a layer of radiation-sensitive material (resist).Prior to this imaging step, the substrate may undergo variousprocedures, such as priming, resist coating and a soft bake. Afterexposure, the substrate may be subjected to other procedures, such as apost-exposure bake (PEB), development, a hard bake andmeasurement/inspection of the imaged features. This array of proceduresis used as a basis to pattern an individual layer of a device, e.g. anIC. Such a patterned layer may then undergo various processes such asetching, ion-implantation (doping), metallization, oxidation,chemo-mechanical polishing, etc., all intended to finish off anindividual layer. If several layers are required, then the wholeprocedure, or a variant thereof, will have to be repeated for each newlayer. Eventually, an array of devices will be present on the substrate(wafer). These devices are then separated from one another by atechnique such as dicing or sawing, whence the individual devices can bemounted on a carrier, connected to pins, etc. Further informationregarding such processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN0-07-067250-4, incorporated herein by reference.

For the sake of simplicity, the projection system may hereinafter bereferred to as the “projection lens”; however, this term should bebroadly interpreted as encompassing various types of projection system,including refractive optics, reflective optics, and catadioptricsystems, for example. The radiation system and projection system mayinclude components operating according to any of these design types fordirecting, shaping or controlling the projection beam of radiation, andsuch components may also be referred to below, collectively orsingularly, as a “lens”. Further, the lithographic apparatus may be of atype having two or more substrate tables (and/or two or more masktables). In such “multiple stage” devices the additional tables may beused in parallel, or preparatory steps may be carried out on one or moretables while one or more other tables are being used for exposures. Dualstage lithographic apparatus are described, for example, in U.S. Pat.No. 5,969,441 and PCT patent application WO 98/40791, incorporatedherein by reference.

It has been proposed to immerse the substrate in the lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the final element ofthe projection system and the substrate. The point of this is to enableimaging of smaller features since the exposure radiation will have ashorter wavelength in the liquid. (The effect of the liquid may also beregarded as increasing the effective NA of the system and alsoincreasing the depth of focus.)

However, submersing the substrate or substrate and substrate table in abath of liquid (see, for example, U.S. Pat. No. 4,509,852, herebyincorporated in its entirety by reference) means that there is a largebody of liquid that must be accelerated during a scanning exposure. Thisrequires additional or more powerful motors and turbulence in the liquidmay lead to undesirable and unpredictable effects.

One of the solutions proposed is for a liquid supply system to provideliquid on only a localized area of the substrate and in between thefinal element of the projection system and the substrate (the substrategenerally has a larger surface area than the final element of theprojection system). One way which has been proposed to arrange for thisis disclosed in PCT patent application WO 99/49504, hereby incorporatedin its entirety by reference. As illustrated in FIGS. 2 and 3, liquid issupplied by at least one inlet IN onto the substrate, preferably alongthe direction of movement of the substrate relative to the finalelement, and is removed by at least one outlet OUT after having passedunder the projection system. That is, as the substrate is scannedbeneath the element in a −X direction, liquid is supplied at the +X sideof the element and taken up at the −X side. FIG. 2 shows the arrangementschematically in which liquid is supplied via inlet IN and is taken upon the other side of the element by outlet OUT which is connected to alow pressure source. In the illustration of FIG. 2 the liquid issupplied along the direction of movement of the substrate relative tothe final element, though this does not need to be the case. Variousorientations and numbers of in- and outlets positioned around the finalelement are possible, one example is illustrated in FIG. 3 in which foursets of an inlet with an outlet on either side are provided in a regularpattern around the final element.

SUMMARY

As system resolution is improved, it becomes increasingly difficult andexpensive to control lens aberrations and focus. The introduction of animmersion liquid in an immersion lithography apparatus has made the taskmore difficult because the optical properties of the liquid are complexand sensitive to small variations in temperature and contaminantconcentration, both of which may change with time and position.

Accordingly, it would be advantageous, for example, to control moreefficiently the optical performance of an immersion lithographyprojection system.

According to an aspect of the invention, there is provided alithographic projection apparatus, comprising:

an illumination system configured to condition beam of radiation;

a support structure configured to hold a patterning device, thepatterning device configured to pattern the beam of radiation accordingto a desired pattern;

a substrate table configured to hold a substrate;

a projection system configured to project the patterned beam onto atarget portion of the substrate;

a liquid supply system configured to supply a liquid to a space throughwhich the beam of radiation passes, the liquid having an opticalproperty that can be tuned; and

a tuner configured tune the optical property.

Imaging radiation may be influenced by one or more portions of liquidencountered prior to the substrate. By providing a tuner to tune one ormore optical properties of these one or more portions and, in anembodiment, use them as a liquid lens(es), it is possible to achieveflexible and dynamic control over imaging performance of thelithographic apparatus. The nature of liquid allows tuning modes thatare not possible with a conventional solid lens or other opticalelement.

An embodiment of the invention may be applied to both immersion andnon-immersion lithographic projection apparatus. Where an embodimentrelates to an immersion lithographic apparatus, the liquid supply systemof the apparatus may configured to at least partly fill a space betweenthe projection system and the substrate with a liquid, and the opticalproperty of the liquid in that space may be tuned. An advantage of usingthe liquid in the space between the projection system and the substrateis that no new volume of liquid needs to be introduced into theapparatus. Additionally, one or more optical properties of that liquidare brought under control, thus removing a need for extensiveadjustments to the projection system to allow for that liquid. As anexemplary embodiment, the liquid may be used to compensate for one ormore specific problems in the projection system. This approach has anadvantage of reducing the need for complex and costly features such asinternal lens manipulators (e.g. Z-manipulators, ALE-manipulators),which might otherwise be required to tune the projection system. Wheresuch lens manipulators are still required, an embodiment of the presentinvention may reduce the range through which they are required tooperate. The liquid may also provide an alternative to using calciumfluoride (CaF₂) for image/lens color correction.

The tuner may be arranged to control spatial dependence of the opticalproperty of the liquid in the space, creating an uniform offset orspatially varying optical property profile. One or more anamorphicimaging effects (e.g. astigmatism offset, asymmetric lens magnification)may be compensated by creating an anamorphic optical property profile(i.e. a profile wherein an optical property is different along twoorthogonal directions). This configuration can be used to compensate alens heating induced effect.

The tuner may be arranged to provide and control a time-varying opticalproperty of the liquid in the space. Changing a temperature profile withtime, for example, coordinated with scanning movement of the substraterelative to the projection system, can induce a lateral refractive indexvariation which can be used to compensate image tilt/curvature and/ordistortion effects.

The tuner may comprise a liquid temperature controller configured tocontrol a temperature profile, and thereby one or more propertiesincluding a refractive index profile, an absorptivity profile, or both,of the liquid in the space. The refractive index profile affects thepath the radiation takes through the liquid and can thus be used tocontrol one or more geometric features of the image such as focus and/oraberration. Temperature provides a highly flexible means of control.Controlling the temperature profile may also affect one or more dynamicproperties of the liquid by influencing viscosity and/or by introducingconvective currents.

The temperature controller may comprise one or more heat exchangersconfigured to establish a homogeneous or non-uniform temperature profilewithin the liquid in the space. Each heat exchanger can act to add heatto the liquid or to remove heat from the liquid.

In an embodiment, the temperature controller may comprise a plurality ofindependent heat exchangers arranged at different heights, radii and/orangles relative to an axis lying in a plane substantially parallel tothe substrate.

The one or more heat exchangers may be arranged to add heat to or removeheat from, but not exchange liquid with, the liquid in the space. One ormore heat exchangers thus arranged may comprise an element which isimmersed in the liquid and maintained at a temperature higher or lowerthan that of the liquid according to whether or not it is requiredrespectively to add or remove heat.

In an embodiment, the one or more heat exchangers may be arranged to addheat to or remove heat from, and exchange temperature conditioned liquidwith, the liquid in the space. One or more heat exchangers that do notexchange liquid with the liquid in the space rely on thermal conductionand convection currents to transport heat, which may lead to delays andunpredictability. By designing the one or more heat exchangers to createcurrents of temperature controlled liquid, the temperature profile maybe adjusted more quickly and accurately. As an exemplary embodiment, theone or more heat exchangers may be arranged in pairs, with a firstelement of each pair adding temperature conditioned liquid and a secondelement removing liquid. Each pair may further be arranged to be alignedin a plane substantially parallel to the plane of the substrate. In thisway, more efficient heat transfer may be achieved. In addition, anuniform controlled flow of liquid substantially parallel to thesubstrate may be provided that allows more predictable and homogeneousoptical properties by reducing convection currents, turbulence and thelike.

The one or more heat exchangers may be coupled with the liquid supplysystem for effecting the exchange of temperature conditioned liquid.This arrangement may be cost effective from a manufacturing perspectivesince the liquid supply system may already be arranged to supply acontrolled flow of liquid, for example, to a space between theprojection system and the substrate.

The tuner may comprise a liquid pressure controller configured tocontrol the pressure, and thereby one or more properties including therefractive index and/or absorptivity, of the liquid in the space. Theuse of pressure has an advantage of high stability and predictability.

The tuner may comprise a liquid geometry controller configured tocontrol a shape of the liquid in the space. The liquid geometrycontroller may operate in combination with the liquid pressurecontroller to vary one or more imaging properties of the liquid. Varyingthe shape of the liquid in the space in this manner allows flexibletuning and may provide a highly stable liquid lens environment.

The liquid geometry controller may control the thickness of the liquidin the space in a direction substantially parallel to the axis of afinal element of the projection system. Increasing the relativethickness of the liquid in this way may be used to control sphericalaberration, for example. This mode has an advantage of providing anadditional means to compensate spherical aberration offset, which cannormally be adjusted only over a limited range. For example,Z-manipulators eventually cause cross talk to other aberrations.

The tuner may comprise a liquid composition controller configured tocontrol the composition, and thereby one or more properties includingthe refractive index and/or absorptivity, of the liquid in the space.

The liquid composition controller may comprise one or more particleexchange configured to add impurity ions to and/or remove impurity ionsfrom the liquid in the space. The liquid composition controller may becoupled with the liquid supply system to provide one or more purityconditioned influxes of liquid.

The liquid composition controller may be arranged to replace a firstliquid in the space with a second liquid of different composition.Water, ethanol, acetone and benzoate are examples of substances that maybe used for either of the first or second liquids. In an embodiment,completely refreshing the liquid in the space provides increased controland scope for image manipulation.

The lithographic projection apparatus may further comprise one or moreliquid sensors configured to measure, as a function of position and/ortime, a property of the liquid in the space including any one or more ofthe following: temperature, pressure, boundary geometry, composition,refractive index and absorptivity. Additionally, the apparatus maycomprise a device configured to correct the focus of the apparatus as afunction of the refractive index profile of the liquid in the space, asmeasured by the one or more liquid sensors. A variation in focus issignificantly dependent on a variation of the refractive index of theliquid. By concentrating on a significant physical property, this devicemay improve the efficiency with which focus in the lithographicapparatus may be controlled.

In an embodiment, the apparatus may comprise a device configured tocorrect an exposure dose of the apparatus as a function of theabsorptivity profile of the liquid in the space, as measured by the oneor more liquid sensors. A variation in radiation intensity reaching thesubstrate is significantly dependent on a variation in the absorptivityof the liquid. By concentrating on a significant physical property, thisdevice may improve the efficiency with which exposure dose in thelithographic apparatus may be controlled

An approach of adjusting one or more optical properties of theprojection system to compensate for a liquid in a space between theprojection system and the substrate without making reference to in situmeasurement of one or more properties of the liquid, requiresexploration of a large parameter space and may therefore be timeconsuming and costly. Liquids for immersion lithography typically havevarious physical properties, including dynamic ones caused by systemflow and convection, that each influence the optical performance indifferent ways. According to an embodiment of the invention when appliedto a liquid in a space between the projection system and the substrate,combining in situ measurement of one or more liquid properties with aknowledge of how each property influences a particular aspect of theoptical performance of the projection system allows more efficienttuning of the projection system.

The tuner may be arranged to create an optical effect includingspherical aberration and/or field curvature. This feature may be used tocompensate for spherical aberration and/or field curvature originatingin the projection system, and thus obviate the need for additionalinternal lens manipulators or other adjustment devices.

The tuner may comprise a computer controller configured to calculate therequired size of correction to one or more optical properties of theprojection system and/or the liquid based on a measured property. Thisapproach obviates the need for extensive experimental tests to determinehow the system may respond to adjustment of a refractive index and/orabsorptivity profile. The computer controller may obtain an estimate forsuch a response via a computer model of the projection system and theliquid (which may or may not be simplified) that provides exact ornumerical solution of relevant physical equations

According to a further aspect of the invention, there is provided adevice manufacturing method, comprising:

providing a liquid in a space through which a beam of radiation passes;

tuning an optical property of the liquid in the space; and

projecting the beam of radiation as patterned by a patterning deviceonto a target portion of a substrate.

According to an aspect of the invention, there is provided alithographic projection apparatus, comprising:

an illumination system configured to condition beam of radiation;

a support structure configured to hold a patterning device, thepatterning device configured to pattern the beam of radiation accordingto a desired pattern;

a substrate table configured to hold a substrate;

a projection system configured to project the patterned beam onto atarget portion of the substrate;

a lens formed from a liquid and having an optical property that can betuned; and

a tuner configured tune the optical property.

Although specific reference may be made in this text to the use of theapparatus according to the invention in the manufacture of ICs, itshould be explicitly understood that such an apparatus has many otherpossible applications. For example, it may be employed in themanufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, liquid-crystal display panels,thin-film magnetic heads, etc. The skilled artisan will appreciate that,in the context of such alternative applications, any use of the terms“reticle”, “wafer” or “die” in this text should be considered as beingreplaced by the more general terms “mask”, “substrate” and “targetportion”, respectively.

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation, including ultravioletradiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm).

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic projection apparatus according to anembodiment of the invention;

FIG. 2 depicts a liquid supply system for supplying liquid to the areaaround the final element of the projection system according to anembodiment of the invention;

FIG. 3 depicts the arrangement of inlets and outlets of the liquidsupply system of FIG. 2 around the final element of the projectionsystem according to an embodiment of the invention;

FIG. 4 depicts a projection apparatus comprising a temperature profilecontroller according to an embodiment of the invention;

FIG. 5 depicts a projection apparatus comprising a liquid compositioncontroller according to an embodiment of the invention;

FIG. 6 depicts a projection apparatus comprising a refractive indexmeasurement device configured to measure the refractive index profile ofthe immersion liquid according to an embodiment of the invention;

FIG. 7 depicts a schematic arrangement for the refractive index sensorof FIG. 6;

FIG. 8 depicts a projection apparatus comprising an absorptivitymeasurement device configured to measure the absorptivity profile of theimmersion liquid according to an embodiment of the invention;

FIG. 9 depicts a schematic arrangement for an absorptivity sensoraccording to an embodiment of the invention;

FIG. 10 depicts a projection apparatus comprising liquid pressure andliquid geometry controllers according to an embodiment of the invention;

FIG. 11 depicts a liquid lens comprising a planar pellicle according toan embodiment of the invention;

FIG. 12 depicts a liquid lens comprising a deformed constrained pellicleaccording to an embodiment of the invention; and

FIG. 13 depicts a liquid lens wherein the liquid is contained betweentwo pellicles according to an embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic projection apparatusaccording to a particular embodiment of the invention. The apparatuscomprises:

a radiation system Ex, IL, configured to supply a projection beam PB ofradiation (e.g. DUV radiation), which in this particular case alsocomprises a radiation source LA;

a first object table (mask table) MT provided with a mask holderconfigured to hold a mask MA (e.g. a reticle), and connected to a firstpositioner configured to accurately position the mask with respect toitem PL;

a second object table (substrate table) WT provided with a substrateholder configured to hold a substrate W (e.g. a resist-coated siliconwafer), and connected to a second positioner configured to accuratelyposition the substrate with respect to item PL;

a projection system (“projection lens”) PL (e.g. a refractive system)configured to image an irradiated portion of the mask MA onto a targetportion C (e.g. comprising one or more dies) of the substrate W.

As here depicted, the apparatus is of a transmissive type (e.g. has atransmissive mask). However, in general, it may also be of a reflectivetype, for example (e.g. with a reflective mask). Alternatively, theapparatus may employ another kind of patterning device, such as aprogrammable mirror array of a type as referred to above.

The source LA (e.g. an excimer laser) produces a beam of radiation. Thisbeam is fed into an illumination system (illuminator) IL, eitherdirectly or after having traversed conditioning means, such as a beamexpander Ex, for example. The illuminator IL may comprise adjustingmeans AM for setting the outer and/or inner radial extent (commonlyreferred to as σ-outer and σ-inner, respectively) of the intensitydistribution in the beam. In addition, it will generally comprisevarious other components, such as an integrator IN and a condenser CO.In this way, the beam PB impinging on the mask MA has a desireduniformity and intensity distribution in its cross-section.

It should be noted with regard to FIG. 1 that the source LA may bewithin the housing of the lithographic projection apparatus (as is oftenthe case when the source LA is a mercury lamp, for example), but that itmay also be remote from the lithographic projection apparatus, theradiation beam which it produces being led into the apparatus (e.g. withthe aid of suitable directing mirrors); this latter scenario is oftenthe case when the source LA is an excimer laser. The current inventionand claims encompass both of these scenarios.

The beam PB subsequently intercepts the mask MA, which is held on a masktable MT. Having traversed the mask MA, the beam PB passes through theprojection system PL, which focuses the beam PB onto a target portion Cof the substrate W. With the aid of the second positioner (and aninterferometric measuring device IF), the substrate table WT can bemoved accurately, e.g. so as to position different target portions C inthe path of the beam PB. Similarly, the first positioner can be used toaccurately position the mask MA with respect to the path of the beam PB,e.g. after mechanical retrieval of the mask MA from a mask library, orduring a scan. In general, movement of the object tables MT, WT will berealized with the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which are not explicitlydepicted in FIG. 1. However, in the case of a stepper (as opposed to astep-and-scan apparatus) the mask table MT may just be connected to ashort stroke actuator, or may be fixed.

The depicted apparatus can be used in two different modes:

1. In step mode, the mask table MT is kept essentially stationary, andan entire mask image is projected at one time (i.e. a single “flash”)onto a target portion C. The substrate table WT is then shifted in the Xand/or Y directions so that a different target portion C can beirradiated by the beam PB;2. In scan mode, essentially the same scenario applies, except that agiven target portion C is not exposed in a single “flash”. Instead, themask table MT is movable in a given direction (the so-called “scandirection”, e.g. the Y direction) with a speed ν, so that the projectionbeam PB is caused to scan over a mask image; concurrently, the substratetable WT is simultaneously moved in the same or opposite direction at aspeed V=Mν, in which M is the magnification of the projection system PL(typically, M=¼ or ⅕). In this manner, a relatively large target portionC can be exposed, without having to compromise on resolution.

FIGS. 2 and 3 depict a liquid supply system according to an embodimentof the invention and have been described above. Another liquid supplysystem solution according to an embodiment of the invention is a liquidsupply system with a seal member which extends along at least a part ofa boundary of the space between the final element of the projectionsystem and the substrate table. The seal member is substantiallystationary relative to the projection system in the XY plane thoughthere may be some relative movement in the Z direction (in the directionof the optical axis). A seal is formed between the seal member and thesurface of the substrate. In an embodiment, the seal is a contactlessseal such as a gas seal. Such a system is disclosed in U.S. patentapplication no. U.S. Ser. No. 10/705,783, hereby incorporated in itsentirety by reference. Other liquid supply systems may be employedaccording to embodiments of the invention including, without limitation,a bath of liquid.

FIGS. 4 to 10 show a liquid in a space through which a beam of radiationpasses according to embodiments of the invention in which an immersionliquid, used for immersion lithography, is used as the liquid. In anembodiment, the liquid in the space forms a liquid lens. A liquid supplysystem supplies liquid to an imaging-field reservoir 12 between theprojection system PL and the substrate W. In an embodiment, the liquidis chosen to have a refractive index substantially greater than 1,meaning that the wavelength of the projection beam is shorter in theliquid than in air or a vacuum, allowing smaller features to beresolved. It is well known that the resolution of a projection system isdetermined, inter alia, by the wavelength of the projection beam and thenumerical aperture of the system. The presence of the liquid may also beregarded as increasing the effective numerical aperture.

The reservoir 12 is bounded at least in part by a seal member 13positioned below and surrounding the final element of the projectionsystem PL. The seal member 13 extends a little above the final elementof the projection system PL and the liquid level rises above the bottomend of the final element of the projection system PL. The seal member 13has an inner periphery that at the upper end closely conforms to thestep of the projection system or the final element thereof and may,e.g., be round. At the bottom, the inner periphery closely conforms tothe shape of the image field, e.g. rectangular but may be any shape.

Between the seal member 13 and the substrate W, the liquid can beconfined to the reservoir by a contact-less seal 14, such as a gas sealformed by gas provided under pressure to the gap between the seal member13 and the substrate W. The liquid may be arranged to be circulated orremain stagnant.

FIG. 4 illustrates an embodiment of the invention in which a tunercomprises a temperature profile controller 24. The temperature profileof the liquid influences predominantly the refractive index profile butlocalized heating/cooling may also influence properties such asabsorptivity and viscosity. The temperature profile controller 24 maycomprise an array of heat exchangers (22,23), which are capable ofheating or cooling the liquid. The heat exchangers may work by contactmeans only (acting locally), or act to supply a flow of temperatureconditioned liquid. In the example shown in FIG. 4, an array of heatexchangers (22,23) provides temperature conditioned liquid via inlets 22and outlets 23 arranged in pairs. Liquid may be made to circulate in aclosed circuit 26 from temperature profile controller 24 through heatexchanger inlets 22 into the reservoir 12 and then back into the closedcircuit 26 via the heat exchanger outlets 23.

The result in this embodiment is a horizontal flow of temperatureconditioned liquid that may support axial (i.e. parallel to the axis ofthe final element of the projection system PL) temperature gradientswith reduced convection currents. The arrangement shown is appropriatefor maintaining such axial temperature gradients. However, radial (froman extension of the axis of the final element of the projection systemPL) temperature gradients may be created or controlled via an analogousarrangement of heat exchangers arranged at different radii, and morecomplex currents may be handled by locating inlets and outlets atdifferent azimuthal angles (i.e. angles relative to a fixed direction ina plane parallel to the substrate W). It is also possible to provide atime-varying temperature profile. This may be done in cooperation withscanning movements of the substrate W and a lateral refractive index canbe induced, along with accompanying image tilt/curvature and distortioneffects.

The inlets 22 and outlets 23 may be coupled with a liquid supply systemsuch as that depicted in FIG. 3. In this example, four groups of ductsare arranged, each angularly spaced from its neighbors by 90°. However,any number of ducts (inlets/outlets) may be used at varioustemperatures, pressures, heights and angular positions for the purposesof tuning one or more optical properties of the immersion liquid.

FIG. 5 shows a further embodiment wherein the tuner comprises a liquidcomposition controller 30. The liquid composition controller 30 may addor remove impurity ions from the immersion liquid in order to influenceproperties such as the refractive index profile or absorptivity. In theexample shown, a single particle exchanger 28 is shown but a pluralityof particle exchangers 28 may be arranged around the reservoir 12 if itis required to create impurity concentration gradients. Additionally oralternatively, one or more particle exchangers 28 may be arranged tocontrol impurities that arise predominantly from particular areas of thereservoir 12 boundary, such as near the substrate W. Again, as for thetemperature profile controller 24, the liquid composition controller 30may be coupled with a liquid supply system such as that depicted in FIG.3.

Additionally or alternatively, a liquid of a different composition maybe added. The second liquid may be mixed with liquid already in thespace or be arranged to replace the original liquid. Examples of liquidsthat may be used include: water, ethanol, acetone and benzoate.

In each case herein, the operation of the tuner may be controlled by acomputer 20 that calculates the required change in the physicalparameter in question via an abstract computer model of the projectionapparatus.

The tuner may be used to create spherical aberration and/or fieldcurvature effects in the liquid in the space. Refractive index changesof between several ppm (parts/million) and several hundred ppm may beused to create such effect(s). For water at 22° C., the rate of changeof refractive index with temperature, dn/dT, is 100 ppm/K. Therefore,changing the refractive index in steps of 50 ppm would requiretemperature steps of 0.5 K. For a typical lens design (variations wouldbe expected between systems of different numerical aperture), this wouldresult in 10-20 nm focus steps and about 1 nm Z9 spherical aberration.The influence of contamination will typically yield approximately 1 ppmchange in refractive index for a 1 ppm change in impurity concentration.For acetone in water the effect is stronger, with an index changemeasured at 10 ppm for a 1 ppm addition of acetone.

FIG. 6 depicts a projection apparatus comprising a measurement device 2configured to measure the refractive index profile of the liquidaccording to an embodiment of the invention. Refractive index sensors16, connected to the device 2, are arranged around the sides of theliquid reservoir 12. Such an arrangement is advantageous where the axialvariation in refractive index is required. The radial variation may bedetermined by positioning sensors 16 at different radii. Sensors thatmeasure through the projection system be used for this purpose.

FIG. 7 shows a schematic arrangement for a refractive index sensor 16.Small quantities of liquid are extracted from the reservoir 12 via thetesting inlet 1 to fill a testing chamber 3. Well collimated light froma light source 5 is arranged to pass through a control medium 7 of knownrefractive index at a fixed angle to the interface between the controlmedium reservoir 9 and the testing chamber 3. The light source 5 may bea low power laser, for example. Light passes through the control medium7 and the immersion liquid in the chamber 3 and is detected by aposition sensitive optical sensor 11 (see example beam path 15). Theangle to the normal is calculated and the refractive index extractedusing Snell's Law.

FIG. 8 depicts a projection apparatus comprising a measurement device 4configured to measure the absorptivity profile of the liquid accordingto an embodiment of the invention. Absorptivity sensors 18 are arrangedin a pairwise fashion around the sides of the liquid reservoir 12 withone element of each sensor pair acting as transmitter and the other as areceiver. The sensors are arranged to be level with each other (in aplane substantially parallel to that of the substrate). The absorptivityis derived by measuring the light attenuation due to propagation acrossthe reservoir 12 of a light beam directed from the transmitter of asensor pair 18 to the matching receiver of the sensor pair 18. Thearrangement depicted is appropriate for measuring axial variations inthe absorptivity profile and for establishing the average overallabsorptivity. Sensors 18 may be arranged at different radii to measureany radial dependence in the absorptivity and/or arranged to measurethrough the projection system.

The absorptivity may also be measured by individual sensors, whichallows more localized measurements of the absorptivity. FIG. 9 depictsan arrangement for such a sensor 32. Here, a small quantity of immersionliquid is removed from the reservoir 12 into an absorptivity testingchamber 34. The absorptivity is derived by monitoring signal attenuationbetween a transmitter 36 and receiver 38.

The measurement device 2, 4 may also comprise one or more sensorsconfigured to measure primary properties such as pressure, temperature,boundary geometry and/or composition. Calibration of one or more ofthese properties may be carried out by reference to sensors forming partof the lithographic apparatus (e.g. focus, aberration and/or dosesensors). Focus, aberration and/or dose sensors may be integrated into awet substrate stage. However, in order to generate useful information,optical measurements using these sensors have to be performed at theimaging wavelength. Therefore, in an embodiment, such measurements aremade offline (i.e. not during imaging) so as not to create strayradiation that could damage the image.

In those sensors described above that extract immersion liquid from thereservoir 12, a mechanism may also be included to purge and replenishthe liquid sample.

In FIGS. 6 and 8, the refractive index measurement device 2 andabsorptivity measurement device 4 are coupled respectively to one ormore devices configured to correct the focus 8 and/or exposure dose 10of the projection apparatus via a computer 20. The computer 20calculates, based on the measured property(ies), what changes to thefocus and/or exposure dose need to be made. This calculation may becarried out based on a feedback mechanism, with a PID(proportional-integral-differential) controller to ensure optimalconvergence of the focus and/or exposure dose towards target value(s).Alternatively or additionally, it may be efficient to utilize afeed-forward arrangement using one or more sensors that are alreadypresent in the substrate holder such as a transmission image sensor(TIS), a spot sensor and an integrated lens interferometer at scanner(ILIAS). Alternatively or additionally, the computer may calculate theappropriate correction(s) based on an abstract mathematical model of theprojection system and immersion liquid. An advantage of the abovearrangements is that they explicitly take into account the physicalinfluence of each property of the immersion liquid. In the examplesdescribed, the absorptivity of the liquid is recognized to besignificant predominantly in relation to exposure dose, while therefractive index profile is recognized to be significant in relation tofocus. Other physical properties may be treated in an analogous way. Forexample, one or dynamic effects linked with motion of the immersionliquid may also affect focus, exposure dose and/or other performancerelated features of the projection apparatus. These one or more effectsmay also be tackled via computer modeling using a similar algorithm asused to model the influence of liquid absorptivity and/or refractiveindex.

One or more optical properties of the liquid in the space may also bevaried by changing the geometry of the liquid. FIG. 10 shows anembodiment wherein the thickness of the liquid (as measured in adirection parallel to the axis of the final element of the projectionsystem) is varied. In this embodiment, a liquid geometry controller 19coordinates the operation of a liquid pressure controller 31 and asecond-component pressure controller 21. The space between the finalelement of the projection system and the substrate is filled with aliquid 12 and a second component 25, which may be a gas such as air. Theliquid and the second component may be constrained within the space byan upper seal member 17 and the contact-less seal 14. The thickness ofthe liquid, meaning the thickness of the liquid 12, is governed by therelative pressures of the liquid 12 and the second component 25,controlled in turn by the liquid pressure controller 31 andsecond-component pressure controller 21. The second component 25 neednot be a gas and may be chosen to be a liquid with a differentcomposition to the first. The relative amounts of the two componentscontained in the space between the final element of the projectionsystem PL and the substrate W may be manipulated to control the positionof the interface between the two and therefore one or more opticalproperties such as spherical aberration.

In an alternative or additional operational mode, the liquid pressurecontroller 31 may be operated independently to control the pressure ofthe liquid 12 and/or any flow of liquid in the space.

FIGS. 11 and 12 show embodiments wherein a pellicle 27 (e.g. a foil ofsolid transparent material such as glass) is provided as an interface tothe liquid in the space on a side of the liquid nearer the final elementof the projection system PL. The pellicle may be laterally unconstrained(FIG. 11) in which case, in an arrangement such as that shown in FIG.10, a planar interface is achieved, the pellicle acting to improve theoptical smoothness of the interface and reduce unwanted scattering.

Alternatively, as shown in FIG. 12, the pellicle may be formed from amaterial that can be deformed and be constrained in such a way that animbalance of pressure on either side of the pellicle causes deformation.In FIG. 12, a concave deformation is formed due to an overpressure inthe liquid 12. As a further variation, the thickness and material of thepellicle 27 may be adjusted to provide further image manipulation.

Further possible variations include non-symmetrical deformation ofeither or both of the predominant interfaces to the liquid 12, such asby tilting one with respect to the other. In the arrangement in FIG. 11,for example, a device may be provided to tilt the pellicle 27.

The above embodiments have shown the liquid in the space formed fromimmersion liquid. However, the space may be anywhere in the beam path.As an example, FIG. 13 shows an alternative embodiment wherein theliquid 12 is constrained between two pellicles 35 and 37. The shape ofthe liquid formed in this way may be varied by adjusting the pressure ofthe liquid 12 via an inlet 33. Either or both of the pellicles 35 and 37may be arranged to be flexible or rigid. A tuner may used to tune theoptical property by, for example, configuring the shape of the liquid bycontrolling pressure and/or the shape of the pellicle(s).

Further, the final element of the projection system PL may consist of aplane parallel plate 29. The mounting of this plate may be such that itcan move towards the substrate W, causing focus offset and/or sphericalaberration offset. In addition, the plate 29 may be tilted, which leadsto focus tilt and/or spherical aberration tilt. This may occur duringscanning movements where a pressure gradient in the liquid 12 isestablished over the surface of the plate 29 (this may depend on how theplate 29 is secured to the rest of the projection system PL). Focus tiltmay cause focus drilling (FLEX) and this may be manipulated bydeliberately controlling the tilt of the plate 29. On the other hand,spherical aberration offset may be manipulated by controlling theoverpressure of the liquid in the space, which affects the position ofthe plate 29 relative to the rest of the projection system.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The description is not intended to limit theinvention.

1-69. (canceled)
 70. A lithographic projection apparatus, comprising: asupport structure configured to hold a patterning device, the patterningdevice configured to pattern the beam of radiation according to adesired pattern; a substrate table configured to hold a substrate; aprojection system configured to project the patterned beam onto a targetportion of the substrate; a liquid supply system configured to supply aliquid to a space through which the beam of radiation passes, the liquidhaving an optical property that can be tuned; and a tuner configured tocontrol spatial dependence of the optical property of liquid in thespace.
 71. The apparatus of claim 70, wherein the tuner comprises aliquid temperature controller configured to control a temperature, andthereby one or more properties including the refractive index,absorptivity, or both, of liquid in the space.
 72. The apparatus ofclaim 71, wherein the temperature controller comprises a heat exchangerconfigured to establish a non-uniform temperature profile within theliquid in the space.
 73. The apparatus of claim 71, wherein thetemperature controller comprises a plurality of independent heatexchangers arranged at different heights, radii, angles, or anycombination of the foregoing, relative to an axis lying in a planesubstantially parallel to the substrate.
 74. The apparatus of claim 70,wherein the tuner comprises a liquid pressure controller configured tocontrol the pressure, and thereby one or more properties including therefractive index, absorptivity, or both, of liquid in the space.
 75. Theapparatus of claim 70, wherein the tuner comprises a liquid geometrycontroller configured to control a shape of the liquid.
 76. Theapparatus of claim 75, wherein the liquid geometry controller isconfigured to control a thickness of the liquid in a directionsubstantially parallel to the axis of a final element of the projectionsystem.
 77. The apparatus of claim 70, wherein the tuner comprises aliquid composition controller configured to control the composition, andthereby one or more properties including the refractive index,absorptivity, or both, of the liquid in the space.
 78. The apparatus ofclaim 70, configured to correct a focus of the apparatus as a functionof the refractive index profile of the liquid in the space, as measuredby a liquid sensor configured to measure, as a function of position,time, or both, a property of the liquid in the space.
 79. The apparatusof claim 70, configured to correct an exposure dose of the apparatus asa function of the absorptivity profile of the liquid in the space, asmeasured by a liquid sensor configured to measure, as a function ofposition, time, or both, a property of the liquid in the space.
 80. Theapparatus of claim 70, wherein the tuner is arranged to create anoptical effect including spherical aberration, field curvature, or both.81. The apparatus of claim 70, wherein the tuner is arranged to createan anamorphic imaging effect by providing an anamorphic optical propertyprofile.
 82. The apparatus of claim 81, wherein the anamorphic imagingeffect includes astigmatism offset, asymmetric lens magnification, orboth.
 83. A device manufacturing method, comprising: providing a liquidin a space through which a beam of radiation passes; controlling spatialdependence of an optical property of the liquid in the space; andprojecting the beam of radiation as patterned by a patterning deviceonto a target portion of a substrate.
 84. The method of claim 83,wherein tuning comprises controlling a temperature, and thereby one ormore properties including the refractive index, absorptivity, or both,of liquid in the space.
 85. The method of claim 84, wherein controllingthe temperature comprises establishing a non-uniform temperature profilewithin the liquid in the space.
 86. The method of claim 84, whereincontrolling the temperature comprises establishing different homogeneousor non-uniform temperature profiles within the liquid in the space atdifferent heights, radii, angles, or any combination of the foregoingrelative to an axis lying in a plane substantially parallel to thesubstrate.
 87. The method of claim 83, wherein tuning comprisescontrolling the pressure, and thereby one or more properties includingthe refractive index, absorptivity, or both, of liquid in the space. 88.The method of claim 83, wherein tuning comprises controlling a shape ofthe liquid in the space.
 89. The method of claim 88, wherein controllingthe shape comprises controlling a thickness of the liquid in the spacein a direction substantially parallel to the axis of a final element ofa projection system used to project the patterned beam.
 90. The methodof claim 83, wherein tuning comprises controlling the composition, andthereby one or more properties including the refractive index,absorptivity, or both, of liquid in the space.
 91. The method of claim83, comprising correcting a focus as a function of the refractive indexprofile of liquid in the space, based on a property of liquid in thespace measured as a function of position, time, or both.
 92. The methodof claim 83, comprising correcting an exposure dose as a function of theabsorptivity profile of liquid in the space, based on a property ofliquid in the space measured as a function of position, time, or both.93. The method of claim 83, wherein tuning comprises creating an opticaleffect including spherical aberration, field curvature, or both.
 94. Themethod of claim 83, wherein tuning comprises creating an anamorphicimaging effect by providing an anamorphic optical property profile. 95.The method of claim 94, wherein the anamorphic imaging effect includesastigmatism offset, asymmetric lens magnification, or both.
 96. Alithographic projection apparatus comprising: a support structureconfigured to hold a patterning device, the patterning device configuredto pattern the beam of radiation according to a desired pattern; asubstrate table configured to hold a substrate; a projection systemconfigured to project the patterned beam onto a target portion of thesubstrate; a lens formed from a liquid and having an optical propertythat can be tuned, or a liquid supply system configured to supply aliquid to a space through which the beam of radiation passes, the liquidhaving an optical property that can be tuned, or both the lens and theliquid supply system; and a tuner configured to spatially vary anoptical property of the liquid.
 97. The apparatus of claim 96, whereinthe tuner comprises a liquid temperature controller configured tocontrol a temperature, and thereby one or more properties including therefractive index, absorptivity, or both, of the liquid.
 98. Theapparatus of claim 97, wherein the temperature controller comprises aheat exchanger configured to establish a non-uniform temperature profilewithin the liquid.
 99. The apparatus of claim 97, wherein thetemperature controller comprises a plurality of independent heatexchangers arranged at different heights, radii, angles, or anycombination of the foregoing, relative to an axis lying in a planesubstantially parallel to the substrate.
 100. The apparatus of claim 96,wherein the tuner comprises a liquid pressure controller configured tocontrol the pressure, and thereby one or more properties including therefractive index, absorptivity, or both, of the liquid.
 101. Theapparatus of claim 96, wherein the tuner comprises a liquid geometrycontroller configured to control a shape of the liquid.
 102. Theapparatus of claim 96, wherein the tuner comprises a liquid compositioncontroller configured to control the composition, and thereby one ormore properties including the refractive index, absorptivity, or both,of the liquid.
 103. The apparatus of claim 96, configured to correct afocus of the apparatus as a function of the refractive index profile ofthe liquid in the space, as measured by a liquid sensor configured tomeasure, as a function of position, time, or both, a property of theliquid in the space.
 104. The apparatus of claim 96, configured tocorrect an exposure dose of the apparatus as a function of theabsorptivity profile of the liquid in the space, as measured by a liquidsensor configured to measure, as a function of position, time, or both,a property of the liquid in the space.
 105. The apparatus of claim 96,wherein the tuner is arranged to create an optical effect includingspherical aberration, field curvature, or both.
 106. The apparatus ofclaim 96, wherein the tuner is arranged to create an anamorphic imagingeffect by providing an anamorphic optical property profile.
 107. Theapparatus of claim 106, wherein the anamorphic imaging effect includesastigmatism offset, asymmetric lens magnification, or both.